Why Did People Create the Great Serpent Mound?/info

Introduction

The Great Serpent Mound (33AD01) is a monumental effigy earthwork located in Adams County, Ohio, extending approximately 411 metres (1,348 feet) along a ridge overlooking Brush Creek. The mound depicts a sinuous serpent with a coiled tail and an open mouth apparently enclosing or ingesting an oval form. Constructed of yellow clay and ash over a prepared base, the effigy averages approximately 1.2 metres in height and 6 metres in width. It is the largest surviving effigy mound in the world, and one of the most intensively studied earthworks in North America.

The site presents two principal archaeological problems that have driven sustained scholarly debate: the question of authorship — whether the mound was built by the Adena culture (ca. 800 BCE – 100 CE) or the Fort Ancient culture (ca. 1000–1650 CE) — and the question of function, particularly the significance of the effigy's apparent astronomical alignments with solstice and lunar standstill events. These questions are interrelated, since cultural attribution determines the interpretive framework within which the monument's purpose is assessed.

Serpent Mound was first formally investigated by Frederic Ward Putnam of Harvard's Peabody Museum in the 1880s, who undertook restoration work that saved the site from agricultural destruction but also introduced ambiguity into the stratigraphic record. The site was inscribed on the National Register of Historic Places and designated a National Historic Landmark. Its nomination to the UNESCO World Heritage List has been under active consideration.

What we know

The Great Serpent Mound is situated on a geological anomaly: the Serpent Mound impact structure, a cryptoexplosion feature approximately 8 kilometres in diameter, formed by either a meteorite impact or explosive volcanism during the Palaeozoic era. The serpent effigy follows the crest of a ridge within this disturbed geology, overlooking a loop of Brush Creek. The relationship between the effigy and the underlying geological feature has prompted speculation about whether the builders were aware of the landscape's unusual character, though no evidence directly demonstrates such awareness.

The effigy's form consists of a tightly coiled tail, seven sinuous curves of the body, and an open-mouthed head oriented toward the northwest. The oval form at the head has been variously interpreted as an egg being swallowed, a symbolic representation of the sun, or a separate but associated feature. No burials, artifacts, or structures have been found within the serpent effigy itself, distinguishing it from nearby conical mounds that do contain Adena burial deposits.

The dating controversy centres on two sets of radiocarbon determinations. Fletcher, Cameron, Lepper, Wymer, and Pickard (1996) obtained radiocarbon dates from charcoal samples within the mound fill that yielded calibrated dates of approximately 1070 CE, associating the mound with the Fort Ancient culture. This attribution was surprising, as the site had been assumed to be Adena since Griffith's analysis of artifacts from adjacent conical mounds in the 1940s.

In 2014, Herrmann, Romain, Monaghan, and Burks challenged the Fort Ancient attribution by obtaining new radiocarbon dates from the base of the mound. Their results yielded dates of approximately 320 BCE, consistent with Early Woodland / Adena period construction. They proposed that the Fletcher et al. dates reflected a Fort Ancient era repair or renovation, not original construction. This reinterpretation was published formally in 2017, initiating a vigorous exchange of rejoinders in the Midcontinental Journal of Archaeology that continued through 2019.

The astronomical alignments of the serpent were first identified by Clark and Marjorie Hardman in 1987, who demonstrated that the oval-and-head area is aligned with the summer solstice sunset azimuth. William F. Romain subsequently proposed a more elaborate scheme of six lunar alignments corresponding to the curves of the serpent's body, with individual coils marking the azimuthal positions of the moon's major and minor standstill rising and setting points. If confirmed, these alignments would represent the integration of solar and lunar astronomical knowledge into a single monumental design.

The site rests within a broader landscape of Adena and Hopewell earthwork construction in the Ohio Valley, including the nearby Tremper Mound, Mound City, and the Newark Earthworks. The Great Serpent Mound is distinctive within this corpus because it is an effigy rather than a geometric or conical burial mound, and because it contains no known interments.

The serpent iconography has connections to widespread Indigenous cosmological traditions. Archaeologist Brad Lepper has proposed that the effigy may depict "Toothy Mouth," a Great Serpent figure from Dhegihan Siouan origin narratives associated with the underworld. The serpent as a cosmological figure appears in the visual traditions of many eastern North American cultures, particularly in the context of the tripartite cosmological division into Upper World, This World, and Beneath World.

Classification of theories

A. Plausible explanations (supported by evidence, not refuted):

1. Ceremonial / cosmological monument. The absence of burials within the effigy, its monumental scale, and its serpentine iconography strongly suggest a ceremonial or cosmological function. The serpent figure is deeply embedded in Indigenous cosmological traditions of eastern North America.

1. Astronomical alignment marker. The summer solstice sunset alignment of the head/oval is well documented by multiple researchers. Romain's lunar standstill alignments are plausible but more complex and less unanimously accepted.

1. Adena construction with Fort Ancient renovation. Herrmann and Romain's 2014/2017 radiocarbon evidence supports original construction in the Early Woodland period (~320 BCE), with a subsequent repair or renovation in the Fort Ancient period (~1070 CE). This two-phase model accommodates both sets of radiocarbon dates.

B. Possible explanations (raised by researchers, less well supported):

1. Fort Ancient construction. Fletcher et al.'s original 1996 dates associate the mound with the Fort Ancient period. Lepper and colleagues continue to defend this interpretation, arguing that Herrmann et al.'s dates may reflect pre-existing soil carbon rather than construction activity.

1. Integrated solar-lunar calendar. The combination of Hardman's solstice alignment and Romain's six lunar alignments would represent a sophisticated merged astronomical calendar, but the lunar alignment claims require further independent verification.

1. Response to the 1054 CE supernova or Halley's Comet. Some researchers have proposed that the mound was built in response to the 1054 CE supernova (visible as a bright celestial event) or the 1066 CE appearance of Halley's Comet, consistent with the Fort Ancient radiocarbon dates.

C. Highly unlikely but argued by some:

1. Extraterrestrial or Atlantean origin. Fringe theories attributing the mound to non-Indigenous builders have no evidentiary support and reflect a tradition of racist pseudoarchaeology that denies Indigenous intellectual and engineering accomplishment.

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Research Papers

Landmark Studies

1. Fletcher, Robert, Terry Cameron, Bradley T. Lepper, Dee Anne Wymer, and William Pickard. "Serpent Mound: A Fort Ancient Icon?" Midcontinental Journal of Archaeology, vol. 21, no. 1, 1996, pp. 105–143. Full text: https://www.semanticscholar.org/paper/Serpent-Mound-:-A-Fort-Ancient-icon-Fletcher-Cameron/4db83608e6928de744074f53fb053b0e18512fe6 [open access — Semantic Scholar]

[Agent-generated summary] The first radiocarbon dating study of the Serpent Mound effigy itself, based on charcoal samples from within the mound fill, yielding calibrated dates of approximately 1070 CE and attributing construction to the Fort Ancient culture rather than the Adena culture as had been assumed for decades.

Full-text notes: Fletcher et al.'s paper fundamentally disrupted the accepted chronology of the Great Serpent Mound. Prior to their work, the site's attribution to the Adena culture rested on associative evidence: the proximity of Adena-period conical burial mounds and the stylistic similarity of the serpent form to other Adena-era earthworks. Fletcher's team obtained the first radiocarbon dates from the effigy itself, based on charcoal recovered from controlled excavations within the mound fill.

The calibrated dates clustered around 1070 CE, corresponding to the Fort Ancient period — a Late Prehistoric culture that was not traditionally associated with monumental earthwork construction. The authors argued that this attribution was consistent with Fort Ancient artistic traditions, which included serpent iconography, and with the broader Late Prehistoric cultural context in the Ohio Valley. The paper also documented the mound's astronomical alignments with solstice and equinox events, proposing that these reflected Fort Ancient astronomical knowledge.

The study's methodology was subsequently challenged on the grounds that the dated charcoal may have been introduced during repair or renovation rather than initial construction, and that the mound fill may contain carbon from multiple periods.

2. Romain, William F., and Edward W. Herrmann. "Radiocarbon Dates Reveal Serpent Mound Is More than Two Thousand Years Old." Midcontinental Journal of Archaeology, vol. 42, no. 3, 2017, pp. 201–222. DOI: 10.1080/01461109.2017.1381956 Full text: https://www.tandfonline.com/doi/abs/10.1080/01461109.2017.1381956 [publisher paywall]

[Agent-generated summary] Romain and Herrmann present new radiocarbon dates from the base of the Serpent Mound, arguing that the effigy was originally constructed during the Early Woodland period (approximately 320 BCE) by the Adena culture, with the Fort Ancient-era dates reflecting a later repair or renovation episode.

Full-text notes: This paper directly challenged the Fort Ancient attribution established by Fletcher et al. (1996). Romain and Herrmann obtained radiocarbon samples from a buried A-horizon beneath the mound base — a stratigraphic position that, if properly identified, would predate the mound's construction. The calibrated dates clustered around 320 BCE, placing original construction firmly within the Adena cultural tradition.

The authors proposed a two-phase model: initial Adena construction followed by Fort Ancient renovation approximately 1,400 years later. This model accommodated both sets of radiocarbon dates and explained the presence of both Adena and Fort Ancient cultural indicators at the site. The paper situated the Serpent Mound within the broader Adena earthwork tradition, arguing that its form and function are more consistent with Adena cosmological practices than with Fort Ancient cultural patterns.

The work reignited the dating debate, prompting a series of replies and rejoinders that occupied multiple issues of the Midcontinental Journal of Archaeology through 2019.

3. Hardman, Clark, and Marjorie H. Hardman. "The Great Serpent and the Sun." Ohio Archaeologist, vol. 37, no. 3, 1987, pp. 34–40. Full text: Not available online. [link unverified — may require institutional access]

[Agent-generated summary] The Hardmans' paper established the first documented astronomical alignment at the Great Serpent Mound, demonstrating that the oval-to-head area of the serpent effigy is oriented toward the summer solstice sunset azimuth.

Full-text notes: This paper introduced archaeoastronomy to the study of the Great Serpent Mound. The Hardmans surveyed the orientation of the serpent's head and the enclosed oval, finding that the sight line from the oval through the open mouth points toward the position of the setting sun on the summer solstice. The alignment is accurate to within approximately one degree, a precision consistent with the scale and construction method of the earthwork.

The solstice alignment has been confirmed by subsequent researchers and is now the least controversial astronomical claim associated with the site. It provides the foundation for more elaborate alignment schemes proposed by later researchers, particularly Romain's lunar standstill hypothesis.

4. Romain, William F. "Serpent Mound and the Milky Way." Time and Mind, vol. 15, no. 4, 2022. DOI: 10.1080/1751696X.2022.2148127 Full text: https://www.researchgate.net/publication/366024376_Great_Serpent_Mound_and_the_Milky_Way [open access — ResearchGate]

[Agent-generated summary] Romain proposes that the Great Serpent Mound encodes alignments not only to solstice and lunar standstill events but also to the Milky Way, arguing that the effigy represents a cosmological map integrating celestial and terrestrial realms.

Full-text notes: This later paper by Romain extends his astronomical analysis beyond solar and lunar alignments to include the Milky Way as a celestial referent. Drawing on ethnographic evidence from multiple Indigenous traditions in which the Milky Way is identified as a "Path of Souls" — a celestial road traversed by the dead — Romain argues that the serpent effigy's sinuous form mirrors the Milky Way's arc across the sky at specific times of year.

The proposal is more speculative than Romain's earlier lunar standstill work, as the Milky Way alignment depends on identifying the entire form of the effigy as a celestial map rather than on discrete cairn-to-cairn sight lines. Nonetheless, the paper contributes to a growing body of work that interprets monumental earthworks as embodiments of Indigenous cosmological narratives rather than as narrowly functional astronomical instruments.

5. Putnam, Frederic Ward. "The Serpent Mound of Ohio." Century Magazine, vol. 39, no. 6, 1890, pp. 871–888. Full text: https://archive.org/details/centuryillustrat39teleunse [open access — Internet Archive]

[Agent-generated summary] Putnam's original popular account of his excavation and restoration of the Great Serpent Mound in the 1880s, which saved the site from agricultural destruction and established it as one of the first preserved archaeological sites in North America.

Full-text notes: Putnam's work was foundational in both archaeological and preservation terms. His excavations documented the mound's construction from yellow clay and ash over a prepared surface, and his restoration work — involving the removal of accumulated soil and vegetation and the re-shaping of the mound to its presumed original contours — established the visual form of the serpent as it appears today. Putnam's intervention was explicitly preservationist: he organised the purchase of the site to prevent its destruction by ploughing.

However, Putnam's restoration also introduced complications for subsequent dating efforts, as the re-shaping of the mound surface may have disturbed stratigraphic relationships. His published accounts, while detailed for their era, reflect the interpretive frameworks and racial assumptions of late nineteenth-century American archaeology. The site's subsequent acquisition by the Ohio Historical Society (now Ohio History Connection) ensured its long-term preservation.

6. Lepper, Bradley T., and Tod A. Frolking. "Debating the Age of Serpent Mound: A Reply to Romain and Herrmann's Rejoinder to Lepper Concerning Serpent Mound." Midcontinental Journal of Archaeology, vol. 44, no. 1, 2019, pp. 76–86. DOI: 10.1080/01461109.2018.1507806 Full text: https://www.tandfonline.com/doi/abs/10.1080/01461109.2018.1507806 [publisher paywall]

[Publisher abstract] The debate over the age of Serpent Mound (33AD01) is important because without a cultural context it is impossible to make meaningful statements about what this monumental effigy mound might have meant to its builders. In this response to Romain and Herrmann's rejoinder, we clarify the provenience of the samples, which yielded the radiocarbon dates that contribute to our argument for a post–Late Woodland age for the effigy.

Full-text notes: Lepper and Frolking's reply represents the Fort Ancient camp's most detailed response to Romain and Herrmann's Adena reattribution. They argue that the radiocarbon dates obtained by Herrmann et al. from the mound base may reflect pre-existing disseminated soil carbon rather than charcoal associated with construction activity. Lepper contends that the stratigraphic identification of a buried A-horizon is uncertain, and that the spatial context of the dated samples does not definitively demonstrate a pre-construction origin.

The exchange is significant not only for the specific dating question but also for its illustration of the methodological challenges inherent in dating earthen monuments that have been subject to restoration, erosion, and bioturbation over millennia. The debate remains unresolved in formal terms, though the Adena attribution has gained increasing acceptance among most North American archaeologists.

Recent Studies

7. Romain, William F., and Edward W. Herrmann. "Serpent Mound in its Woodland Period Context: Second Rejoinder to Lepper." Midcontinental Journal of Archaeology, vol. 44, no. 1, 2019, pp. 62–75. DOI: 10.1080/01461109.2018.1511155 Full text: https://www.tandfonline.com/doi/abs/10.1080/01461109.2018.1511155 [publisher paywall]

[Agent-generated summary] Romain and Herrmann's second rejoinder places the Serpent Mound within the broader Woodland period context of effigy mound construction, arguing that an Adena-period construction date is consistent with the regional pattern of monumental earthwork building during the Early and Middle Woodland periods.

Full-text notes: This paper strengthens the case for Adena construction by expanding the comparative framework. Romain and Herrmann survey the corpus of Woodland-period earthworks in the Ohio Valley and upper Midwest, demonstrating that monumental effigy construction is well attested in the Early and Middle Woodland periods. They argue that attributing the Serpent Mound to the Fort Ancient period places it as an anomaly within Fort Ancient cultural patterns, which did not otherwise include large effigy earthwork construction.

The contextual argument is persuasive as a supplement to the radiocarbon evidence, though it relies on negative evidence — the absence of Fort Ancient effigy mounds elsewhere — which can be overturned by future discoveries.

8. Lepper, Bradley T., Robert F. Boszhardt, James R. Duncan, and Carol Diaz-Granados. "Effigy Mounds and Rock Art of Midcontinental North America: Shared Iconography, Shared Stories." Midcontinental Journal of Archaeology, vol. 47, no. 1, 2022, pp. 3–34. DOI: 10.1177/0197693121996728 Full text: https://journals.sagepub.com/doi/abs/10.1177/0197693121996728 [publisher paywall]

[Agent-generated summary] A comparative iconographic study placing the Great Serpent Mound within the broader context of effigy mound and rock art traditions across midcontinental North America, exploring shared cosmological narratives encoded in monumental landscape art.

Full-text notes: This collaborative study examines the serpent motif and related iconographic elements across a wide geographic and temporal range, arguing that monumental effigy construction participated in shared narrative traditions that crossed cultural and linguistic boundaries. The paper's treatment of the Great Serpent Mound emphasises its cosmological dimensions — particularly the serpent as a figure associated with the Beneath World in the tripartite cosmological framework documented ethnographically across eastern North America.

The study is valuable for moving beyond the dating debate to address questions of meaning and function that apply regardless of whether the Adena or Fort Ancient attribution proves correct.

9. Romain, William F. "Arguments for the Age of Serpent Mound." Cambridge Archaeological Journal, vol. 28, no. 3, 2018, pp. 433–450. DOI: 10.1017/S0959774318000069 Full text: https://www.cambridge.org/core/journals/cambridge-archaeological-journal/article/abs/arguments-for-the-age-of-serpent-mound/52318DAD1CF14EA426D1B6D286A91930 [publisher paywall]

[Publisher abstract] Serpent Mound, in northern Adams County, Ohio, is the world's largest effigy mound. The effigy has been central to archaeological debate over its age and cultural affiliation. Some archaeologists consider it to have been built by the Adena culture around 300 BC, while others contend it was built by the Fort Ancient culture around AD 1100.

Full-text notes: Romain's Cambridge Archaeological Journal paper presents the Adena case to an international audience, providing a comprehensive review of the dating evidence and the comparative cultural context. The paper is notable for its detailed treatment of the stratigraphic arguments, including a rebuttal of the claim that the basal radiocarbon dates reflect disseminated soil carbon rather than construction-era charcoal.

Romain also addresses the astronomical alignments in this paper, arguing that the solstice and lunar standstill alignments are more consistent with the Adena cultural tradition — which has well-documented astronomical knowledge at sites like the Newark Earthworks — than with the Fort Ancient tradition. This integrative approach, linking dating evidence to cultural context and astronomical function, represents the most complete statement of the Adena attribution case.

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Current Discussions

1. Ohio History Connection — "Who Built Serpent Mound?" https://www.ohiohistory.org/who-built-serpent-mound/ [open access] — A public-facing overview of the dating debate, written by Brad Lepper, acknowledging that the Adena attribution has gained wide acceptance while noting the provisional character of all archaeological knowledge.

1. The Debrief — "Ohio's Serpent Mound Still Fuels Debate" (March 2026). https://thedebrief.org/ohios-serpent-mound-still-fuels-debate-as-haunting-questions-remain-about-americas-most-mysterious-earthwork/ [open access] — A recent longform treatment summarising the state of the debate and noting ongoing efforts toward UNESCO World Heritage inscription.

1. Archaeological Conservancy — "The Serpent Mound Debate" (2017). https://web.archive.org/web/20231201061453/https://www.archaeologicalconservancy.org/serpent-mound-debate/ [open access — archived] — A balanced summary of the competing chronological interpretations, aimed at an informed general audience.

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Future Research Directions

Additional targeted radiocarbon dating of the Serpent Mound, employing contemporary Bayesian modelling techniques and focused on unambiguously identified stratigraphic contexts, could resolve the dating debate. Particular attention to the interface between the mound base and the original ground surface — using microstratigraphic analysis and micromorphology — would help determine whether the earliest dates reflect pre-construction soil or construction-era activity.

Ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) surveys of the mound's interior could reveal internal construction phases, compaction horizons, and any subsurface features without physical excavation. Such surveys have been successfully applied to other Woodland-period earthworks and could provide non-invasive evidence for the proposed two-phase construction model.

Systematic archaeoastronomical verification of Romain's lunar standstill alignments, using high-resolution topographic data and precise horizon modelling, would either strengthen or weaken the case for the mound as an integrated solar-lunar calendar. This work would benefit from collaboration between archaeoastronomers and specialists in computational modelling of ancient horizon profiles, accounting for vegetation, erosion, and changes in the tree line since the mound's construction.

Research into the geological awareness of the mound's builders — specifically whether the placement of the effigy on the Serpent Mound impact structure reflects intentional selection of a geologically anomalous landscape — could be advanced through comparative studies of earthwork placement across the Ohio Valley, testing whether monumental constructions are statistically associated with geological anomalies.

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Summary of Existing Research and Public Opinion

The Great Serpent Mound has been a subject of archaeological inquiry since the 1840s, and the intensity of research has increased markedly since Fletcher et al.'s 1996 radiocarbon dates disrupted the longstanding Adena attribution. The subsequent dating debate between the Fort Ancient and Adena camps, played out over multiple rounds of publication in the Midcontinental Journal of Archaeology and the Cambridge Archaeological Journal, represents one of the most sustained and methodologically detailed chronological controversies in North American archaeology.

The current scholarly consensus leans toward original Adena construction (ca. 320 BCE) with Fort Ancient renovation (ca. 1070 CE), though the Fort Ancient-only interpretation retains defenders. The astronomical alignments — particularly the solstice sunset alignment of the head — are widely accepted, while the more elaborate lunar standstill scheme proposed by Romain is intriguing but not universally endorsed.

Public engagement with the site is substantial. The Great Serpent Mound attracts significant tourist visitation and has become a symbol of Indigenous intellectual achievement in the popular imagination. It has also been subject to appropriation by New Age and pseudoarchaeological movements, which sometimes attribute the mound to non-Indigenous builders or invoke supernatural explanations. Mainstream archaeological communication has increasingly emphasised the Indigenous origins and cosmological sophistication of the monument as a counterweight to such narratives.

The ongoing UNESCO World Heritage nomination process has elevated the site's international profile and focused attention on the need for accurate and respectful interpretation. The nomination dossier emphasises both the monument's archaeological significance and its ongoing cultural importance to descendant Indigenous communities.

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Where Do I Come In?

Archaeologists and geoarchaeologists with expertise in radiocarbon dating, Bayesian chronological modelling, and microstratigraphy are directly positioned to contribute to resolving the dating controversy. The Serpent Mound dating debate is fundamentally a question about the stratigraphic provenance of radiocarbon samples, and specialists in soil micromorphology could provide the microscale evidence needed to determine whether the basal dates reflect pre-construction surfaces or construction-era deposits.

Geophysicists and remote sensing specialists can advance knowledge of the mound's internal structure without physical excavation. Ground-penetrating radar and electrical resistivity tomography surveys, calibrated against known earthwork construction techniques from the Ohio Valley, could identify internal construction phases and test the two-phase construction hypothesis non-invasively.

Indigenous scholars, community members, and descendant communities hold knowledge traditions concerning the cosmological significance of the serpent figure and the broader landscape that academic archaeology has only partially engaged. Collaborative research frameworks that position Indigenous knowledge holders as partners rather than subjects represent the most promising approach to understanding the monument's meaning within the cultural systems that produced it.

Citizen scientists and engaged members of the public can contribute by supporting the site's preservation and by advocating for accurate interpretation in public discourse. The Great Serpent Mound is frequently misrepresented in popular media and online spaces; informed public voices that can communicate the actual state of archaeological knowledge serve an important corrective function. Visitors to the site during solstice events can contribute observational data on alignment accuracy under varying atmospheric conditions, adding to the empirical record that supports or refines astronomical interpretations.

Educators and museum professionals can develop interpretive programming that conveys the sophistication of Indigenous earthwork engineering and astronomical knowledge, countering the persistent tendency in popular culture to attribute monumental Indigenous constructions to external or supernatural agencies.